1. Technical Field
The present invention relates to a novel polyimide and a polyimide precursor (polyamic acid) as a precursor thereof, useful as e.g. protective films and insulating films for liquid crystal display devices and semiconductor devices, and as optical waveguide materials for optical communication, excellent in transparency at not only a visible region but also an ultraviolet region even after baking at a high temperature of from 270xc2x0 C. to 350xc2x0 C, and having characteristics such as a low dielectric constant, a low birefringence and a high heat resistance.
2. Background Art
Wholly aromatic polyimide are insoluble in a solvent in general, and by coating a polyimide precursor as a precursor thereof on a substrate by e.g. casting or spin coating, followed by heating at a high temperature, a desired polyimide can be obtained. All such heat resistant aromatic polyimides present deep amber and are colored in general.
Polyimides are widely used as protective materials or insulating materials for liquid crystal display devices and semiconductor devices by virtue of the high mechanical strength, heat resistance, insulating properties and solvent resistance. They are used also as optical waveguide materials for optical communication. However, developments in these fields have been remarkable in recent years, and increasingly high levels of properties have been required for the materials to be used in such fields. Namely, they are expected not only to be excellent in heat resistance, but also to have various performances depending upon application.
In recent years, protective materials and insulating materials for liquid crystal display devices and semiconductor devices are required not only to have a heat resistance but also to maintain transparency at not only a visible region but also an ultraviolet region after baking at a high temperature of from 270xc2x0 C. to 350xc2x0 C., or to have a low birefringence and a low dielectric constant when formed into a coating film in some cases.
For example, with respect to a buffer coating material as a protective film for a specific semiconductor device, in order to erase memory errors generated during preparation of the element, through-holes are formed by utilizing lithography technology for electrical erasion, and such makes the process complicated. If the buffer coating material has transparency to ultraviolet light, optical erasion by UV irradiation alone becomes possible without formation of through-holes, and the process can be simplified. In such a case, great absorption of ultraviolet light is fatal. Further, in a field of specific optical waveguide materials, materials having not only a high heat resistance but also a small birefringence and a high transparency at an ultraviolet region are desired.
As one method for realizing transparency at a visible region, it is well known to obtain a polyimide precursor by a polycondensation reaction of an aliphatic tetracarboxylic dianhydride with a diamine, followed by imidizing said precursor to produce a polyimide, whereby a polyimide which is relatively less colored and is excellent in transparency can be obtained (JP-B-2-24294, JP-A-58-208322).
It is certain that when a polyimide is prepared by such a known method using an aromatic diamine as a diamine, a polyimide having excellent transparency at a visible region in the vicinity of 400 nm will be obtained, but a great absorption will usually appear at an ultraviolet region in the vicinity of 300 nm, where electron transition absorption of an aromatic ring is present. Further, many of aliphatic tetracarboxylic dianhydrides have a low reactivity in general, and it is thereby difficult to obtain a polymer having a high degree of polymerization unless structurally suitable one is selected.
Further, as a method to reduce absorption at not only a visible region but also an ultraviolet region and to present a coating film having a low birefringence, a polyimide consisting of a combination of an aliphatic tetracarboxylic dianhydride with a specific aliphatic diamine has been proposed (W. Folksen et al., Reactive and Functional polymer, vol. 30, Page 61, 1996). It is certain that a polyimide consisting of this combination is excellent in transparency at not only a visible region but also an ultraviolet region, but a coating film tends to be yellow by baking at a high temperature in the vicinity of 300xc2x0 C., and is poor in heat resistance. Further, basicity of an aliphatic diamine is high as compared with that of an aromatic diamine, and accordingly the aliphatic diamine tends to form a salt with a carboxylic acid generated during polymerization, whereby it is difficult to control solubility, and polymerization may not proceed in some cases, and thus it is difficult to say the method is common.
Under these circumstances, the present invention has been made to provide a novel polyimide excellent in transparency at not only a visible region but also ultraviolet region after baking at a high temperature of from 270xc2x0 C. to 350xc2x0 C., and having characteristics such as a low dielectric constant, a low birefringence and a high heat resistance.
The present inventors have conducted extensive studies to overcome the above problems and as a result, found that a polyimide obtained by imidizing a polyimide precursor comprising a specific diamine having cyclobutanetetracarboxylic dianhydride and a hexafluoropropylidene group in its molecule can achieve the above object. The present invention has been accomplished on the basis of the above discovery.
Namely, the present invention relates to a polyimide precursor having a repeating unit represented by the following general formula (1): 
(wherein R1 is a bivalent organic group constituting a diamine), wherein R1 contains a bivalent organic group constituting a diamine having a hexafluoropropylidene group in its molecule represented by the following general formula (2): 
(wherein A is a hydrogen atom, a linear alkyl group including a methyl group, or a trifluoromethyl group, and n is the number of a substituent on an aromatic ring and an integer of from 1 to 4), and the reduced viscosity is from 0.05 to 5.0 dl/g (in N-methylpyrrolidone at a temperature of 30xc2x0 C., concentration: 0.5 g/dl), and further relates to a polyimide having a repeating unit of the general formula (3): 
(wherein R1 is the same as in the above formula (2)) which is obtained by imidizing said precursor.
Now, the present invention will be explained in further detail below.
A tetracarboxylic component to be used to obtain the polyimide precursor represented by the above general formula (1) of the present invention is cyclobutanetetracarboxylic acid, its dianhydride and its dicarboxylic acid diacid halide.
The diamine component may, for example, be 2,2-bis(3-aminophenyl)hexafluoropropane, 2,2-bis(4-methyl-3-aminophenyl)hexafluoropropane, 2,2-bis(4,5-dimethyl-3-aminophenyl)hexafluoropropane, 2,2-bis(4-trifluoromethyl-3-aminophenyl)hexafluoropropane, 2,2-bis(4,5-bistrifluoromethyl-3-aminophenyl)hexafluoropropane, 2,2-bis(4-aminophenyl)hexafluoropropane, 2,2-bis(3-methyl-4-aminophenyl)hexafluoropropane, 2,2-bis(2,3-dimethyl-4-aminophenyl)hexafluoropropane, 2,2-bis(3-trifluoromethyl-4-aminophenyl)hexafluoropropane or 2,2-bis(2,3-bistrifluoromethyl-4-aminophenyl)hexafluoropropane. They may be used alone or in combination as a mixture of two or more of them.
In order to achieve the effects of the present invention, preferably from 70 mol % to 100 mol % of a polyimide precursor consisting of the above combination is contained so as to obtain the effects of the present invention remarkably also.
As the other tetracarboxylic dianhydride components, tetracarboxylic dianhydrides and their derivatives, commonly used for synthesis of a polyimide, can be used without any problem.
Specific examples thereof include alicyclic tetracarboxylic acids such as 1,2,3,4-cyclopentanetetracarboxylic acid, 2,3,4,5-tetrahydrofurantetracarboxylic acid, 1,2,4,5-cyclohexanetetracarboxylic acid, 3,4-dicarboxy-1-cyclohexyl succinic acid and 3,4-dicarboxy-1,2,3,4-tetrahydro-1-naphthalene succinic acid, their dianhydrides and their dicarboxylic acid diacid halides.
Further, aromatic tetracarboxylic acids such as pyromellitic acid, 2,3,6,7-naphthalene tetracarboxylic acid, 1,2,5,6-naphthalene tetracarboxylic acid, 1,4,5,8-naphthalene tetracarboxylic acid, 2,3,6,7-anthracene tetracarboxylic acid, 1,2,5,6-anthracene tetracarboxylic acid, 3,3xe2x80x2,4,4xe2x80x2-biphenyl tetracarboxylic acid, 2,3,31,4-biphenyl tetracarboxylic acid, bis(3,4-dicarboxyphenyl) ether, 3,3xe2x80x2,4,4xe2x80x2-benzophenone tetracarboxylic acid, bis(3,4-dicarboxyphenyl)sulfone, bis(3,4-dicarboxyphenyl)methane, 2,2-bis(3,4-dicarboxyphenyl)propane, 1,1,1,3,3,3-hexafluoro-2,2-bis(3,4-dicarboxyphenyl)propane, bis(3,4-dicarboxyphenyl)dimethyl silane, bis(3,4-dicarboxyphenyl)diphenyl silane, 2,3,4,5-pyridine tetracarboxylic acid and 2,6-bis(3,4-dicarboxyphenyl)pyridine, and their dianhydrides and their dicarboxylic acid diacid halides; and aliphatic tetracarboxylic acids such as 1,2,3,4-butane tetracarboxylic acid, and their dianhydrides and their dicarboxylic acid diacid halides, may, for example, be mentioned.
Further, one or more of these tetracarboxylic acids and their derivatives may be used in mixture.
As the other diamine components, a primary diamine to be commonly used for synthesis of a polyimide may be mentioned, and they are not particularly limited.
Specific examples thereof include aromatic diamines such as p-phenylenediamine, m-phenylenediamine, 2,5-diaminotoluene, 2,6-diaminotoluene, 4,4xe2x80x2-diaminobiphenyl, 3,3xe2x80x2-dimethyl-4,4xe2x80x2-diaminobiphenyl, 3,3xe2x80x2-dimethoxy-4,4xe2x80x2-diaminobiphenyl, diaminodiphenylmethane, diaminodiphenyl ether, 2,2xe2x80x2-diaminodiphenylpropane, bis(3,5-diethyl-4-aminophenyl)methane, diaminodiphenylsulfone, diaminobenzophenone, diaminonaphthalene, 1,4-bis(4-aminophenoxy)benzene, 1,4-bis(4-aminophenyl)benzene, 9,10-bis(4-aminophenyl)anthracene, 1,3-bis(4-aminophenoxy)benzene, 4,4xe2x80x2-bis(4-aminophenoxy)diphenylsulfone, 2,2-bis[4-(4-aminophenoxy)phenyl]propane, 2,2xe2x80x2-trifluoromethyl-4,4xe2x80x2-diaminobiphenyl and 4,4xe2x80x2-bis(4-diaminophenoxy)octafluorobiphenyl; alicyclic diamines such as bis(4-aminocyclohexyl)methane and bis(4-amino-3-methylcyclohexyl)methane, and aliphatic diamines such as tetramethylenediamine and hexamethylenediamine; as well as diaminocycloxanes such as 
(wherein m is an integer of from 1 to 10). Further, these diamines may be used alone or in combination as a mixture of two or more of them.
As a method to obtain the polyimide precursor of the present invention, specifically, said tetracarboxylic dianhydride and its derivative are reacted and polymerized with the above diamine. The ratio of the molar amount of the tetracarboxylic dianhydride to the total molar amount of the diamine and common diamines, is preferably from 0.8 to 1.2. Like in a usual polycondensation reaction, the polymerization degree of the resulting polymer tends to be large, as the molar ratio becomes close to 1. In such a case, if the polymerization degree is too small, strength of the polyimide film tends to be inadequate. On the other hand, if the polymerization degree is too large, the operation efficiency at the time of formation of the polyimide film tends to be poor in some cases. Accordingly, the polymerization degree of the product in this reaction is preferably from 0.05 to 5.0 dl/g (in N-methylpyrrolidone at a temperature of 30xc2x0 C., concentration: 0.5 g/dl) as calculated as the reduced viscosity of the polyimide precursor solution.
A method of reacting and polymerizing Etetracarboxylic dianhydride with the above diamine is not particularly limited, but it is common to employ a method wherein the above diamine is dissolved in a polar solvent such as N-methylpyrrolidone, and to the solution, the tetracarboxylic dianhydride is added and reacted to synthesize a polyimide precursor. The reaction temperature may be an optional temperature selected within a range of from xe2x88x9220xc2x0 C. to 150xc2x0 C., preferably from xe2x88x925xc2x0 C. to 100xc2x0 C.
As a polymerization method of a polyamic acid, a conventional solution method is suitable. Specific examples of the solvent to be used for the solution polymerization include N,N-dimethylformamide, N,N-dimethylacetamide, N-methyl-2-pyrrolidone, N-methyl caprolactam, dimethyl sulfoxide, tetramethyl urea, pyridine, hexamethylphospholamide and butyrolactone. They may be used alone or in mixture. Further, a solvent in which a polyimide precursor is not dissolved may be added to the above solvent within a range where homogeneous solution is obtained.
The polyimide precursor solution of the present invention may be used as it is, or may be used as an organic solvent-soluble polyimide. A method of obtaining an organic solvent-soluble polyimide is not particularly limited, but it may be obtained by dehydration ring-closure imidization of a polyimide precursor in general.
The polyimide precursor obtained by reacting the tetracarboxylic dianhydride with the diamine, may be kept in a solution and imidized to obtain a solvent-soluble polyimide solution.
To convert the polyimide precursor to a polyimide in the solution, it is common to employ a method wherein dehydration ring closure is carried out by heating. The ring closure temperature by dehydration under heating may be an optional temperature selected within a range of from 100xc2x0 C. to 350xc2x0 C., preferably from 150xc2x0 C. to 350xc2x0 C., more preferably from 270xc2x0 C. to 350xc2x0 C. The reason why the temperature should be at least 270xc2x0 C. is to assure long-term reliability of a coating film by completely converting the polyamic acid into a polyimide and by removing a remaining solvent. Further, it is not necessary to conduct baking at a temperature exceeding 350xc2x0 C. since no further improvement in baking effect will be achieved.
As another method for converting the polyimide precursor to the polyimide, it is possible to carry out ring closure chemically by means of a known dehydration ring-closing catalyst. Further, it is possible to achieve an optional degree of imidization by selecting the temperature or time during dehydration ring closure. The solvent for dissolution is not particularly limited so long as it is capable of dissolving the obtained polyimide, and examples of which include m-cresol, 2-pyrrolidone, N-methylpyrrolidone, N-ethylpyrrolidone, N-vinylpyrrolidone, N,N-dimethylacetoamide, N,N-dimethylformamide and xcex3-butyrolactone.
The polyimide solution thus obtained may be used as it is, or may be precipitated and isolated in a poor solvent such as methanol or ethanol to obtain a polyimide as a powder, or said polyimide powder may be re-dissolved in a proper solvent.
The solvent for re-dissolution is not particularly limited so long as it is capable of dissolving the polyimide, and examples of which include 2-pyrrolidone, N-methylpyrrolidone, N-ethylpyrrolidone, N-vinylpyrrolidone, N,N-dimethylacetoamide, N,N-dimethylformamide and y -butyrolactone.
The above polyimide precursor and the solvent-soluble polyimide may be used as they are, or they may be mixed with one or more polyimide precursor or solvent-soluble polyimide. The mixing ratio may optionally be selected.
The solvent to be used for the solvent-soluble polyimide of the present invention is not particularly limited so long as it is capable of dissolving the polyimide, and examples of which include 2-pyrrolidone, N-methylpyrrolidone, N-ethylpyrrolidone, N-vinylpyrrolidone, N,N-dimethylacetoamide, N,N-dimethylformamide and xcex3-butyrolactone.
Further, even a solvent incapable of dissolving the polymer by itself may be added to the above solvent within a range of not impairing the solubility.
Examples thereof include ethyl cellosolve, butyl cellosolve, ethyl carbitol, butyl carbitol, ethyl carbitol acetate and ethylene glycol.
Further, for the purpose of further improving the adhesion of the polyimide film to a substrate, it is of curse preferred to add an additive such as a coupling agent to the obtained polyimide solution.
This solution is coated on a substrate, and the solvent is evaporated, whereby a polyimide coating film can be formed on the substrate. As the temperature at that time, a temperature of from 100 to 300xc2x0 C. is usually sufficient.
Now, the present invention will be explained in further detail with reference to Examples, but the present invention is by no means restricted thereto. Here, physical properties of the obtained polyimide were evaluated by means of the following apparatuses and methods.
1) Viscosity Measurement
An NMP solution having a solid content of 0.5 wt % was prepared, and measurement was carried out by using an Ubbellohde viscometer at 30xc2x0 C.
2) Ultraviolet Spectrum
UV-3100PC manufactured by Shimadzu Corporation was used.
3) 5% Weight Loss Temperature
Measured by using a thermogravimetric analyzer manufactured by Mac Science at a temperature-increasing rate of 10xc2x0 C./min.
4) Glass Transition Temperature
Measured by using a thermomechanical measuring apparatus manufactured by Mac Science with a load of 5 g at a temperature-increasing rate of 5xc2x0 C./min.
5) Index of Birefringence
By using a prism coupler model 2010 manufactured by Metricon, employing TE and TM direction polarizations at a wavelength of 633 nm with a film thickness of 2xc3x97103 nm, and the difference thereof was obtained.
6) Dielectric constant
Measured by using an AG-4311BLCR meter and SE-70 model electrode for solid, manufactured by Ando Electric Co., Ltd., at 25xc2x0 C. at 100 kHz.